Daddario PW-SIH-01

The sponge is... Well, it cannot hold much water so you will be filling it up quite often.
The case can, however, easily be filled with either orbeez or silica gel depending on if you want to add or remove moisture.
Some people use a soap bar for this as a DIY solution, which is bulkier . This is smaller.

One small negative though is that the lid is on the loose side so if you have anything else than the sponge in it it might be a good idea you secure it a little better. IT might even be, that you are unlucky and the sponge falls out and come in contact with your instrument.
It has not happened to me, at least not yet, but it is a small concern.

I use these in the headstock compartment of my acoustic guitar cases with the planetwaves PW-GH-HTS humidity control & sensor system with the sound hole humidifier taking care of the other end and they serve that purpose very well even though it's not exactly what they were designed for. When I bought my first really good acoustic I used the Planetwaves Humidipack system which is fantastic & has the added bonus of absorbing excess moisture when the humidity level gets to high. I never had a problem with that system & found it excellent, & I have never heard of anyone ever having any problem, but the idea of a pouch of liquid/gel hanging in the sound hole of my pride & joy became to much for my nerves so I switched. I'd like to repeat I have never heard of any problem with the humidipaks leaking, but I just wanted 101% peace of mind. With the PW-SIH-01 all moisture is held by a sponge so there is definitely absolutely no chance of leaks or spillages. Great little product.